This invention concerns a cooled bottom electrode for a direct-current electric furnace for the melting and refining of metallic alloys which are advantageously iron-based.
The invention is applied to direct-current electric furnaces which are used for the melting and refining of metals and which comprise at least one upper electrode inserted into the furnace from above and a plurality of bottom electrodes incorporated in the refractory hearth of the furnace.
The invention concerns an improvement of the structure of the bottom electrodes so as to achieve an improvement and an increase of the efficiency of the cooling action of the bottom electrodes.
This leads to an improvement of the operation of the furnace in terms of productive efficiency and of the working life of the electrodes and prevents possible operational accidents and enables still further advantages to be achieved.
Direct-current electric furnaces typically contain an upper electrode, which generally consists of graphite, is associated with the furnace roof and extends into the furnace, and also contain a plurality of electrodes associated with the hearth of the furnace so as to close the electrical circuit.
In direct-current electric furnaces the bottom electrodes are most likely the most delicate component mainly owing to the fact that they are traversed by currents of a very great intensity and undergo intense thermal stresses.
Various types of these bottom electrodes have been developed, and each type possesses its own specific advantages and drawbacks.
For instance, these bottom electrodes have been embodied in the form of metallic bars incorporated in the refractory hearth of the furnace and extending at their lower end at least partly outside the furnace itself.
The number of these bars and their arrangement, which is advantageously symmetrical in relation to the centre of the furnace, depend on the power of the furnace and on the conformation of its hearth.
According to another type of bottom electrode these metallic bars can be divided into a plurality of billets, which have a very small diameter and are fixed at their lower end to a common plate, which is generally air-cooled and is connected by water-cooled conductors to the electricity supply.
Each electrode unit may consist, instead of billets, of a plurality of metallic fins welded to a common metallic support and arranged in cooperation with other electrode units so as to form a ring which is advantageously concentric with the furnace.
Another approach to their embodiment has the hearth of the furnace consisting of a conductive material for the passage of the direct current through the hearth.
According to the state of the art the electrodes of a bar type can be made of steel and copper or wholly of steel.
The upper part of these bars, as it is in contact with the bath of molten metal, melts down to a certain height.
Depending on the efficiency of the cooling, the bar has an upper liquid part and a lower solid part, the parts being divided by a separation zone.
With this type of bottom electrode the main problem is to develop a cooling system able to ensure along the height of the bar a solid part reaching as high as possible, even under the conditions of the high electrical load conducted by these bottom electrodes.
This is necessary, amongst other reasons, so as to prevent the formation of possible routes of escape for the liquid metal.
Various solutions have been disclosed for achieving a high thermal efficiency of the action to cool the bottom electrodes.
In EP-A-0474883 the bottom electrodes, which consist of metallic rods of a small diameter, are assembled in a plurality of electrode units, each of which includes a common conductor plate to which are fitted all the electrodes of the specific electrode unit. This document discloses the cooling of the bottom electrodes by means of the circulation of a forced draught between the plates to which the electrodes are fitted and the plate fitted below the hearth of the furnace.
U.S. Pat. No. 4,592,066 includes a bottom electrode consisting of a metallic plate inserted centrally into the hearth of the furnace; to the lower surface of the plate is fixed a bar which extends downwards out of the hearth.
The part of the bar outside the furnace is surrounded by a sleeve, in which cooling water is fed.
GB-A-1,162,045 includes a bottom electrode consisting of two parts, an upper part and a lower part connected together. The upper part in contact with the bath of molten metal consists of a metal which is the same as that being melted, whereas the lower part, which is not in contact with the bath, consists of a material possessing properties of high electrical and heat conductivity, such as copper for instance.
According to this document the lower part has the purpose of removing heat from the electrode, and its bottom end, which protrudes out from the hearth of the furnace, can be shaped in various ways, for instance as a plate, so as to increase its radiant surface.
EP-A-0449258 discloses a furnace having bottom electrodes of which the part protruding downwards from the hearth is associated with a cooling-water box connected to means that feed and discharge the cooling water.
None of these systems of the state of the art has been able to ensure the achievement of a sufficient solid level of the steel bar incorporated in the refractory hearth owing to the high thermal resistance provided by the steel part of the bar.